1. Field of the Invention
This invention relates to an inflatable type occupant restraint system, or as it is commonly known an air bag restraint system. Such a system may be installed in an automobile or other vehicle, at least in part on the steering wheel for the protection of the driver and also on the dashboard for passenger protection in the event of a collision. More particularly, this invention relates to an improvement in the means for holding the air bag in such a device.
2. Description of Related Art
During the manufacture of an air bag restraint system numerous components must be installed and checked to produce the finished air bag module. Among the components of the air bag module are the cover, air bag, air bag holder means (annular retainer, module housing, and fasteners), gas generator or inflator, ignition element, and a means to attach the assembly to the body of the automobile. The inflator due to the presence of the gas generant, a pyrotechnic, is the most sensitive component. For safety reasons it is desirable to delay installation of the inflator to as late as possible in the assembly process.
In a vehicle equipped with an air bag safety system, the air bag is instantly inflated in the event of a collision to protect the occupant from injury. The air bag is inflated by the pressure of a gas supplied from an inflator fixed or mounted to the steering wheel, dashboard or other appropriate part of the body of the automobile.
It has been determined that in order to protect a driver during a collision of an automobile the air bag should be inflated within 20 to 40 milliseconds after the initial impact. The high pressure gas, supplied to the air bag, necessary to inflate the air bag within the short period of time required produces forces which tend to pull and separate the air bag from its mounting. In addition, the impact of the occupant against the inflated air bag tends to increase these forces. To resist these strong forces a mounting device of considerable strength must be provided to insure the safety of the occupant.
In a conventional mounting for an air bag, the peripheral region around the outer circumference of the gas inlet opening into the air bag is clamped between a module housing provided around the gas discharge or exit opening and an annular retainer secured to the module housing. The module housing is mounted to the vehicle body, for example, the steering wheel. The annular retainer is bolted or riveted to the module housing.
In another conventional mounting arrangement the air bag is riveted to the annular retainer. The annular retainer with the air bag affixed thereto is then bolted to the module housing and the inflator. The use of rivets to attach the air bag is a major cause of reworking of the inflator module assembly due to the necessity of replacing broken or unfastened rivets. Reworking of the inflator module is not always possible. In these situations the defective modules must be discarded, resulting in lost production and increased cost of manufacturing. The elimination of rivets in the manufacture of the air bag module assembly would eliminate a major cause for reworking and disposal of defective inflator modules.
In the event of a collision, the large forces produced by the inflation of the bag and the impact of the body of the occupant against the bag tend to pull the clamped portion of the bag from the inflator. The force holding the air bag to the inflator is not evenly distributed over the entire area of the abutment between the annular retainer and the clamped portion of the air bag. Instead, the force of holding the air bag to the inflator is concentrated in the shearing resistance of that part of an air bag which has a width corresponding to the diameter of the fastener and a length corresponding to the distance from the fastener to the edge of the gas inlet opening of the air bag. This clamping action of the retainers may be insufficient to prevent separation of the bag from the retainers. Such separation would permit the escape of gas directly into the passenger compartment of the vehicle endangering the safety of the driver and other occupants of the vehicle.
Numerous solutions have been suggested to improve the clamping of the air bag and to increase the shear strength of the air bag between the fastener and the inner edge of the gas inlet opening. An undersized air bag opening which thereby increases the length of bag available to resist the shear stress upon deployment of the air bag is disclosed by Sudou, U.S. Pat. No. 4,183,550. The additional section of the air bag between the annular retainer and the edge of the bag opening is placed between the inner edge of the annular retainer and the inflator housing. This arrangement like previous clamping devices relies on the shear strength of the air bag which lies radially inward from the fastener to overcome the deployment forces. To facilitate the installation of the undersized bag opening between the clamps and the inflator housing the bag must be slit in a number of locations, thereby weakening the air bag by eliminating the circumferential strength at the air bag gas inlet opening.
The use of a chain-stitched stitch at the air bag gas inlet opening to resist the deployment forces applied to the bag is disclosed by Takada, U.S. Pat. No. 4,793,611. This arrangement permits movement of the bag to align the edge of the gas opening against the inner edge of the annular retainer and mounting bracket. As with other state of the art clamping systems this arrangement relies primarily on the shear strength of the bag in proximity to the fasteners.